This invention relates generally to the art of seismic investigation of the ocean bottom and more particularly to an improved method and apparatus for processing acoustic signals to provide data from which interpretations can be made as to the geologic character of the bottom and substrata thereof, and as to effects, such as impulse response and bottom loss, on sonar transmissions.
Seismic exploration of the ocean bottom has long been carried out by explosively generating a low frequency acoustic pulse in the water and then receiving and recording, at some distance by shipboard equipment, the acoustic energy reflections from the various bottom surfaces, strata, and layer boundaries. Interpretive analysis of the recorded acoustic data has yielded information in amounts that would otherwise be virtually impossible to obtain by mechanical soundings and corings alone, although those techniques are still valuable for other purposes.
Such seismic exploration has been considerably refined over the years, particularly in the signal processing, recording, and analyzing of the acoustic returns. One of the presistent problems has been the presence of what are known as secondary or "bubble pulse" returns. The bubble pulses occur as a result of the rapid collapse of the bubble produced by the explosive charge used to generate the primary acoustic signal. These secondary bubble pulse returns can become confused with and mask primary return data. This phenomena is explained in U.S. Pat. No. 2,622,691 to J. R. Ording, which discloses a method for determining the presence of such secondary return signals.
Attempts have been made to eliminate the bubble pulse by caging the explosive shot in a perforate housing that retards the rate of bubble collapse, however the most effective solution to the problem has been to remove the secondary return signals from the recorded data by computer processing of the recorded data. Heretofore, the most effective computer processing technique for that purpose has been to perform a deconvolution of the incident and reflected pulses. This is done by tape recording the acoustic return signals, converting the signals to tape recorded digitized form and feeding them to a digital computer that does a fast Fourier transform on both the incident and the reflected signals. An inverse Fourier transform is then performed on the quotient of the Fourier transform of the reflected signal divided by the Fourier transform of the incident signal. The net result is the transfer function in the time domain or the impulse response in the frequency domain. Digital fast Fourier transform computations of the acoustic signals require complex, specialized digital electronic computers that are very expensive. Moreover, such systems are limited in bandwidth capability.
It is known that an optical lens has the inherent property of producing a Fourier transform of a signal at the focal plane of the lens, and that a beam of light can be diffracted by passage through a fluid that is subjected to acoustic energy at predetermined frequencies. Heretofore such acousto-optic systems have been used for generating holographic displays of sections of a body immersed in the fluid. Examples of this manner of ultrasonic examination of a body are found in U.S. Pat. No. 3,737,573 to L. W. Kessler, and No. 3,745,814 to D. Gabor. The latter utilizes a flexible membrane to confine the body containing acoustically excited fluid and reflected light from the membrane is modulated by deformations thereof. A correcting hologram is used in conjunction with a schlieren stop to compensate for imperfections in the shape of the membrane.